1. Field of the Invention
The disclosures herein relate to a transimpedance amplifier, a circuit mounting structure using the transimpedance amplifier and an optical transceiver.
2. Description of the Related Art
Along with an increase in computing power of computers, high-speed and multichannel interconnection between computers has been developed. For example, parallel optical transceiver modules are paralleled at a channel pitch of 250 μm. Moreover, signal speeds reach 25 Gpbs/ch, and further increasing signal speed is considered.
A photodiode (hereinafter referred to as “PD”) array used in a parallel module includes a cathode and an anode arranged in each channel, and a transimpedance amplifier (hereinafter referred to as “TIA”) array includes a bias terminal and an input terminal arranged in each channel. The cathode is connected to the bias terminal, and the anode is connected to the input terminal.
When connecting the PD to the TIA with a wire, a connection distance is shortened along with the increase in signal speed. However, when a flip chip bonding is used to connect the PD and the TIA, a connection distance is likely to increase to prevent interference in tools while mounting a chip and interference in underfill, which causes characteristic degradation and instability of bias voltage.
Japanese Patent Application Publication No. 2012-142822 discloses an optical receiver and an optical transmitter that are configured to stabilize a signal while implementing a 250 μm pitch by connecting grounded capacitors to cathodes of photodiodes and arranging the capacitors alternately on an upper surface and a back surface of a substrate.
Japanese Patent Application Publication No. 2015-56704 discloses an optical module configured to reduce cross talk between anode wiring arranged at a narrow pitch by electrically connecting each cathode to a backside pattern of a substrate so that cathode potential of each channel is common, and matching the anode wiring to TIA input impedance.
The input impedance of the TIA circuit is impedance relative to the ground potential, and the input impedance cannot be matched in the transmission line with respect to the cathode as disclosed in Japanese Patent Application Publication No. 2015-56704.
On the other hand, when the transmission line is configured with respect to the ground, the characteristic difference between the channels occurs unless the ground terminal is provided for each channel. The ground terminals are preferred to be provided for all of the channels, because the channel closer to the ground terminal is likely to have better channel characteristics.
However, when the ground terminal is provided for each channel in addition to a cathode terminal and an anode terminal, it might be difficult to mount the TIA to a substrate.